A much safer alternative to hydrochloric, sulphuric acid etc is boric acid, often sold as a pharmaceutical, I believe it forms part of many skin preparations. It is not expensive, about £5 for 500gms, and made up as a saturated solution, about 5-7% solution in distilled water at 70-80 C, it lightly etches steel to give a silk grey finish over an hour or two depending on temperature.
It also removes rust and other oxide finishes very gently but completely. I have used it several times to remove the thin brown rust coating on lock plates and actions and although the finish is a bit drab for my taste, it may be what you are looking for. There doesn't appear to be any after rusting, a problem with many stronger acids.
Used on Damascus barrels it will produce a differential etching of the steel and iron to prepare the barrels for browning. I prepare all my damascus barrels (thoroughly plugged of course!) and damascus snapcaps with this solution. After degreasing with 'whiting' I lay the items in 70-80C solution for anything from 30 min to 3 hours depending on the finish I require.
The carbon in the steel remains on the etched surface and just needs wiping off before thorough rinsing and then straight into the browning process.
It is pretty safe on one's skin although I always wear light gloves as it tends to dry one's skin out.
